The present invention relates to a sparkplug having good antifouling properties for use in internal combustion engines.
If the temperature of the tip of the firing leg of the porcelain insulator in a sparkplug is less than 500.degree. C., there is a tendency for carbon, water and other electrically conducting materials to be deposited on the firing surface. Moreover, if the insulation resistance between the electrodes becomes lower than 1 M.OMEGA., a spark cannot easily be formed and normal spark discharge then cannot occur. This phenomenon (misfiring) is apt to occur if electrically conductive materials are deposited on the firing surface of the porcelain insulator as a result of repeated short cycles of idling or running at low speeds or before a new car has been run 1000 km.
An antifouling sparkplug is known that has a coating of heat-resistant, insulating material on the inner surface of the combustion chamber side of the metal casing to prevent short-circuiting between the firing surface of the porcelain insulator and the metal casing. It has been proposed to use silicone oil as such a heat-resistant, insulating material. In such a case, a coating of silicone oil is formed on the inner surface of the metal casing and the firing surface of the porcelain insulator. However, subsequent studies have revealed that a coating of silicone oil alone is not effective in preventing the decrease in insulation resistance. Specifically, silicone has a chain combination --Si--O--Si--O-- which makes the coating gas-permeable. Therefore, if a silicone coating is formed on the inner surface of the metal casing or the firing surface of the porcelain insulator, carbon or water can penetrate therethrough making the silicone coating electrically conductive. As a result, the insulation between the firing surface of the porcelain insulator and the metal casing is impaired thus reducing the antifouling properties of the spark-plug.